The present invention relates to emergency shutdown systems and more particularly to devices and methods for testing and verification of emergency shutdown valves without disrupting process operation and the process coefficient of flow.
The purpose of an emergency shutdown system is to safely shutdown a process that for some reason has exceeded the normal operating envelope for that process. If the process were allowed to continue to exceed the normal operating envelope, personnel may be harmed and facilities may be damaged. The process is normally maintained within its safe operating envelope with the primary control system. If the primary control system should fail, the process may exceed the safe operating envelope where upon the emergency shutdown system would shut off raw materials to the process. The final control element of the emergency shutdown system is the shutdown device, which is usually a valve or damper. Normally this shutdown device can only be tested when a process is intentionally being shut down or after a process has been shut down since the shut down device will function to shutdown the process when the process has exceeded its safe operating envelope. The present invention is directed to controlling valve type shut down devices.
Presently there are test methods in which a change in state of an electrical contact causes a shutdown devices to go to a pre-defined position. These methods require a hardware xe2x80x9cstopxe2x80x9d device to be activated at the pre-defined position to prevent the shutdown device from shutting down the process. These methods also require a change of the state of a contact on the shutdown device in order to confirm that the shutdown device has attained the pre-defined position. These methods totally depend on discrete changes of state of electrical contacts for both activation and confirmation.
Digital control systems have been developed which test the operation and movement of a valve actuator, as set forth in U.S. Pat. No. 6,176,247 B1. Shut down valve operation methods and devices and described in U.S. Pat. No. 6,186,167 B1 which describes a method of moving a shut down valve, during a testing procedure, from either a fully opened or closed position to a partially closed or opened position.
The present invention includes methods for controlling the shutdown device via an analog signal, allowing for the shutdown device to move 50% without affecting the flow rate of the process. By utilizing an analog signal as a feedback of the position of the shutdown device, confirmation of a 50% change in position can be attained without the need for a change in the state of a contact. The actuation and confirmation of a change in position of the shutdown device results in a reliable xe2x80x9con-linexe2x80x9d test of a shutdown device without affecting the process. Furthermore, the present invention utilizes a shut down valve with a valve coefficient of flow (CV) that allows the valve to be moved from a position beyond fully opened to a partially closed position without reducing the valve coefficient of flow below a level equal to the coefficient of flow for the process. Therefore the emergency shut down valve movement can be tested without disrupting the process flow.
The disclosure herein includes four features that make an on-line test of a shutdown device possible and unique:
1. The testing method imparts no reduced affect on the flow of raw material to the process or energy to the process. The shut down testing method utilizes a shutdown device that is larger than the valve or damper that would normally be used to control the raw material flow there through. For example; for a 1.25 inch valve the CV is 46 (valve coefficient of flow). By utilizing a 2.0 inch valve that has a CV of 120, the 2.0 inch valve can be 25% closed before its CV is equal to the CV of a fully opened 1.25 inch valve. By utilizing a ball type valve, and removing the fully-opened stop, the valve may be moved from 25% before fully opened to 25% after fully opened (25% closed) without affecting the necessary raw material flow to the process. The result is a 50% change in position of the valve without affecting the process and a test procedure that test the movement and operation of the shutdown device without affecting the flow of materials there through.
2. The method utilizes an analog signal to cause the shutdown device to move from a 25% before the fully opened position to a pre-defined 25% after fully opened position. The result is a 50% change in analog signal to the shutdown device and a 50% change in the shutdown device, without affecting the flow of raw materials to the process or affecting the flow of energy to the process.
3. The method utilizes a feedback analog signal to confirm that the shutdown device has in fact moved 50%. A failure of the shutdown device to move to the pre-defined position will sound an alarm and display the failure condition. Immediate corrections can be made to the shutdown device to return it to fully functional state. Without an on-line test, a shutdown device that failed to operate correctly can only be corrected after a costly process shutdown.
4. The system can be tested as frequently as desired, thus increasing the reliability of the shutdown device. A continuous process that shuts down once a year can only allow a test of the shutdown device once a year, whereas an on-line test of the shutdown device twice a day results in 720 test per year. There is a greater assurance that the shutdown device will function correctly when it is tested 720 times a year rather than once.
By having tested the shutdown device to 50% of stroke the following has been accomplished:
1. The analog output signal directing the valve to stroke has been tested.
2. The shutdown device hardware (valve) has been tested.
3. The analog input signal indicating shutdown device position has been tested.
Having tested the shutdown device to 50% of stroke, the device can be expected to operate to 100% of stroke on demand.
It is thus an object of the invention to provide a shutdown device (valve or damper) that is significantly larger than the pipe or duct that delivers raw materials to a process, and controlling that shutdown device via an analog signal, to cause the shutdown device to move 50% without affecting the process flow. To utilize an analog signal as a feedback of the position of the shutdown device, confirmation of a 50% change in position can be attained. To confirm the change in position of the shutdown valve without affecting the process.
It is another object of the present invention to enable xe2x80x9con-linexe2x80x9d testing of shutdown valves and controllers, without affecting the process.
It is another object of the present invention to provide methods and apparatus which provides a method of frequent testing of the shutdown device, and improving the reliability of the shutdown system.